Do We Need to Redefine the Four Seasons?

As climate change affects the planet's weather patterns, some parts of the world will see the seasons bleed together. Tune in to learn how we define the seasons now, and how some researchers think that should change.

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Episode Transcript

Speaker 1 (00:02):
Welcome to brain stuff from how Stuff Works. Hey, they're
brain stuff, Lauren Vogal bomb here. If you live in
Earth's middle latitudes, you're accustomed to experiencing four traditional seasons winter, spring, summer,
and autumn, also known as fall. That means that existence
in the two bands of latitude stretching around the planet
from thirty to sixty degrees both north and south of

(00:24):
the traffics, offers a lot more variety weatherwise than on
the equator, where there's basically a hot, dry season and
a hot rainy season. It's a similar story for the
upper latitudes, whose residents get a cold winter with long
dark nights and a slightly less cold summer with longer daylight.
To be technical about it, there are actually two different
ways of defining the seasons, the astronomical definition and the

(00:46):
meteorological definition. The astronomical definition is based upon change in
the length of days that's caused by the relative tilt
of the Earth's axis as it revolves around the sun.
So in that system, winter is the time between the
winter's solstice, the shortest period of daylight of the year,
which occurs around December twenty two, and the vernal or
spring equinox when day and night are roughly equal, which

(01:09):
occurs around March twenty one. Spring then lasts from the
vernal equinox to the summer solstice, which is the longest
daylight period of the year and happens around June two.
Summer runs from then until the autumnal equinox on September
twenty three, another day when light and darkness are equal.
Then autumn continues until the winter solstice. The meteorological method,

(01:29):
on the other hand, has been around since at least
the mid nineteen hundreds and is a lot simpler. The
year is divided into four seasons by calendar months, so
that winter is December January February, spring is March April, May,
summer is June July August, and autumn is September October November.
Study conducted by climate researcher Kevin Trenberth found that the

(01:50):
simpler meteorological definition more closely agreed with observable weather in
the continental regions of the Northern Hemisphere, while the trickier
astronomical definition fit reality better over the oceans in the
southern Hemisphere. Nevertheless, the astronomical definition of the seasons continues
to be the one generally used in the United States.
Trenwerth told us after my article there were a few

(02:11):
places that tried to change, but it petered out. However,
climate change, largely driven by human activity, has been messing
with the traditional concept of four seasons. Scientists have discovered
that as the planet warms up, the tropics have been
expanding by point one to point two degrees of latitude
every decade. That means places that once had four seasons
are shifting to having just two. But even in regions

(02:33):
that still experienced four seasons, weather and temperature patterns have
been altered. Across the United States. The shift from cold
winter weather to warm spring temperatures happens earlier now than
it did in the past, and the period of winter
weather is shorter and generally milder. In the first few
months of seventeen, Some eastern US cities were even beset
by startling summerlike conditions in February, with many reaching or

(02:55):
surpassing their all time temperature records. Climate change induced seasonal
creep a coupled with the overall warming trend, might make
you wonder whether the concept of four seasons may eventually
become obsolete. Trend Birth, who is currently a scientist at
the National Center for Atmospheric Research and who was a
co recipient of the two thousand seven Nobel Peace Prize
for his work in climate studies, doesn't see that happening,

(03:16):
but he says the definition of the seasons may change.
Will still have winter, spring, summer in autumn in the
middle latitudes, just the timing and duration will be different,
he said. A key point, though, is the idea of
four seasons, and the way I think of it is
the two extreme seasons summer and winter, and to transition seasons.
One could redefine the latter to be shorter. In some sense,

(03:37):
the summer is becoming longer. The summer is not just temperature,
but also the character of the weather, more convective thunderstorms, etcetera,
versus winters, more extratropical storms, cold frinds, etcetera. There's one
analysis that suggests summers are now thirteen days longer and
winters are twenty days shorter than they used to be.

(04:00):
Today's episode was written by Patrick J. Keiger and produced
by Tristan McNeil. For more on this and lots of
other atmospheric topics, visit our home planet, how stuff Works
dot com